How can it be impossible to “decrypt” an MD5 hash? [duplicate]

Answer 1

Think about this:

I have a numeric string, say it's "12345678".

I have a hash algorithm, it just returns the sum of all the single numbers, let's call it f()

so, f("12345678") = 1 + 2+ .. + 8 = 36.

Then the question:

known f(x) = 36, is it possible to get the original value of x ?

We can't, because f() is an algorithm causes information loss.

The MD5 is a hash algorithm like f(), but much more complexer.